With recent increases in the efficiency of magnetic recording media, there has been a concomitant demand for higher density recording. This demand has created a renewed interest in CrO.sub.2, in addition to metal and Ba ferrite, as ferromagnetic particles. Attention has been particularly drawn to CrO.sub.2, since CrO.sub.2 has excellent magnetic properties such that (1) saturation magnetization is high; (2) electric resistance is small; (3) demagnetization under pressure is small; (4) the direction of crystal anisotropy axis and the shape of the anisotropy axis are very close; and that (5) Curie point can be varied.
Binders which may be used in such conventional magnetic recording media containing CrO.sub.2 magnetic particles include polyurethane mainly composed of an aliphatic polyester polyol component and a polyisocyanate component, as disclosed, for example, in U.S. Pat. No. 4,568,611.
However, this type of conventional magnetic recording medium containing CrO.sub.2 fine particles and a polyurethane binder has the undesirable defect that since hydrolysis of polyurethane is accelerated under high temperatures and high humidity, electromagnetic properties and running durability are not satisfactory.
Accordingly, a binder having excellent hydrolysis resistance has been required to prepare magnetic recording media which would ensure the above-described advantageous properties of CrO.sub.2 particles.